The present invention relates to turbocharged internal combustion engines and more particularly to such an engine with an exhaust gas recirculation system and a turbocharger compressor recirculation valve to lower turbocharger compressor discharge pressure and thereby assist exhaust gas recirculation flow.
It is known in the art of controlling the generation of undesirable pollutant gases and particulate matter in the operation of internal combustion engines to use an exhaust gas recirculation (EGR) system. EGR systems recirculate exhaust gas into the engine intake air supply reducing the concentration of oxygen, which lowers the combustion temperature within the cylinder and slows the chemical reaction of a combustion process, decreasing the formation of nitrous oxides (NOx). Additionally unburned hydrocarbons contained in exhaust gas are re-introduced into the engine cylinders which further reduces the emission of undesirable hydrocarbons.
In some internal combustion engines and particularly diesel engines a turbocharger is used to increase engine performance. Therein the exhaust gas to be recirculated is diverted upstream of the exhaust gas turbine associated with the turbocharger.
In a high pressure system exhaust gas is pumped directly into the intake manifold. However, under lower speed and high torque conditions, the boost pressure is higher than the exhaust manifold pressure and recirculation of exhaust gases is not possible. A present method of generating EGR flow to correct this problem has been to use various devices such as back pressure valves or restrictive turbines such as a variable nozzle turbine (VNT).
With a variable nozzle turbine its vanes are adjustably closable to restrict exhaust gas flow through the turbine. Thereby turbine inlet pressure is increased to a point where it exceeds the intake manifold pressure as generated by the turbocharger compressor. During the process the exhaust gas that does pass between the vanes of the turbine has a higher velocity, due to the reduced cross-sectional area between the vanes. This increased gas velocity, when it strikes the turbine wheel, causes higher wheel rotational speed. This higher speed on the compressor side causes increased boost or compressor discharge pressures. The overall impact, therefore, is that the very concept used to increase EGR flow by increasing the turbine inlet pressure, also increases the compressor discharge and intake manifold pressure, thereby hindering further increases in EGR flow.
The result is that compressor discharge and intake manifold pressures become unacceptably high in the effort to generate higher EGR flow rates for reduced NOx emissions. These high intake manifold pressures tax the capability of the charge cooling system and associated ducting to withstand the higher pressures and contribute to unacceptably high combustion peak firing pressures. Another unwanted result is that the turbocharger rotation speeds can become unacceptably high during high load engine operation.
The present invention provides an internal combustion engine having means to lower turbocharger compressor discharge and intake manifold pressure allowing an appropriate EGR flow rate to be more readily generated at lower turbocharger rotational speeds and reducing combustion peak firing pressure.
The present invention also provides an intake air bypass that allows turbocharger compressor discharge air to be recirculated back into the turbocharger compressor inlet. A control valve may be provided to control bypass flow.
The present invention optionally provides a pressure relief device for bypassing charge air back to the turbocharger compressor inlet as a function of engine operating conditions. At lower engine loads, the device is closed and EGR systems operate normally. At sufficiently high engine loads, the compressor discharge pressure will rise to the level of the pressure relief devices opening pressure, allowing charge air to be recirculated back to the turbocharger compressor inlet. This recirculated charge air allows the intake manifold pressure to remain low enough to cause the pressure difference between the turbine inlet and the intake manifold to be sufficiently large to allow the proper EGR flow.
In engines using a variable nozzle turbine, a more open VNT vane position allows the turbocharger to rotate at a lower speed which is beneficial to turbocharger durability. The lowering of the intake manifold results also in a decrease in the combustion peak firing pressure, which is beneficial to engine durability.
Accordingly an internal combustion engine in accordance with the invention includes a block having a plurality of cylinders. An intake manifold is fluidly connected to the block for supplying charge air to the cylinders. An exhaust manifold is fluidly connected to the plurality of cylinders for conducting exhaust gas away from the cylinders. A turbocharger including a turbine having an exhaust gas inlet is fluidly connected with the exhaust manifold. The turbocharger also includes a compressor having a compressor air inlet and air outlet. The compressor air outlet is fluidly connected to the intake manifold to pressurize the charge air during high power levels of engine operation.
An EGR bypass is fluidly connected between the turbine exhaust gas inlet and the intake manifold to recirculate a portion of the exhaust gases to the cylinders. An intake air bypass is fluidly connected between the compressor air outlet and the compressor air inlet. The intake air bypass is adapted to recirculate a portion of compressor outlet air back to the compressor air inlet to reduce compressor outlet pressure and aid EGR flow through the EGR bypass to the cylinders during high power operation of the engine.
In one embodiment of the invention the turbocharger includes a variable geometry turbine operable to raise engine exhaust pressure by restricting exhaust gas flow to the turbine. The intake air bypass includes a control valve operable to control bypass flow. The control valve may be a pressure relief valve that is opened by excess pressure from the compressor outlet. Alternatively the control valve may be operated by suitable engine control apparatus.
The engine may include a charge air cooler fluidly connected between the compressor air outlet and the intake manifold for cooling compressed charge air prior to delivery into the cylinders. The intake air bypass may be fluidly connected with an outlet of the charge air cooler to provide cool air to the compressor inlet.
The EGR bypass can be fluidly connected with the intake manifold after the connection of the intake air bypass with the compressor air outlet to avoid recirculating exhaust gas to the compressor air inlet.
The EGR bypass may include a cooler to cool hot exhaust gas prior to entering the intake manifold.
These and other features and advantages of the invention will be more fully understood from the following detailed description of the invention taken together with the accompanying drawings.